Sensor networks are becoming more popular. Radio Frequency Identification (RFID) tags have been used prevalently and those that include sensing capabilities have emerged as a generally inexpensive and effective means of addressing many wireless sensor applications. Purely passive sensors, such as Computational RFID tags (CRFID), when actively interrogated by an RF transceiver/reader, receive energy via the interrogation signal to power themselves up so that they can acquire readings from their sensors. Generally, CRFID tags equipped with one or more sensors require an internal energy source, or an internal reservoir of previously stored energy, to measure and store their acquired information at times other than during active interrogation by a reader.
Next generation sensor networks may be powered by energy harvesting techniques, to avoid requiring batteries. Energy harvesting is the process by which energy is derived from external sources (e.g., solar power, thermal energy, wind energy, salinity gradients, and kinetic energy), captured and stored.
Among the various harvesting options include radio frequency energy (RF). With RF energy harvesting, wireless energy is harvested from a transmitter that is some distance away from devices with RF power harvesting capabilities. Such transmitters can include transmitters that transmit un-modulated RF energy for the specific purpose of serving as a power source, as well as other transmitters that perform a communication function.
One of the more popular forms of RF data communications used today is Wi-Fi communications. Today, most Wi-Fi transmitters and receivers operate in the 2.4 GHz or 5 GHz frequency bands and there are many local area networks that are based on Wi-Fi in which access points enable Wi-Fi clients to gain access to networks such as the Internet. Additionally, other communication standards such as Zigbee and Bluetooth operate in the 2.4 GHz frequency spectrum. Other devices emit energy primarily in the 2.4 GHz Industrial Scientific and Medical (ISM) band such as microwave ovens and medical diathermy machines.